This book traces the position of the United States of America on aggression, beginning with the Declaration of Independence up to 2020, covering the four years of the Trump Administration. The decision of the Assembly of States Parties to the International Criminal Court to activate the Court’s jurisdiction over the crime of aggression in 2018 has added further value to a book concerning the position and practice of one of the most influential states, a global military power and permanent member of the UN Security Council. Organized along chronological lines, the work examines whether, or to what extent, the US position has evolved over time. The book explores how the definition of the crime can impact upon the US, notwithstanding its failure to ratify the Rome Statute. It also shows that the US practice and opinio iuris about the law applicable to the use of force might influence, as it has done in the past, the law itself. The work will be a valuable guide for students, academics and professionals with an interest in International Criminal Law.